The inventor's assignee has proposed in, e.g., U.S. Pat. Nos. 5,243,588 and 5,244,705, and pending U.S. patent application Ser. No. 07/736,046, the disclosures of which are incorporated herein by reference, a technique for compressing a digital audio input signal and recording the resulting compressed recording signal in bursts with a predetermined number of bits of the compressed recording signal as a recording unit.
With this technique, the compressed recording signal is an adaptive differential PCM (ADPCM) audio signal, and a magneto-optical disc is used as the recording medium for recording the compressed recording signal according to the so-called CD-I (CD-Interactive) or CD-ROM XA recording signal format. The compressed recording signal is recorded in bursts on the magneto-optical disc, with, e.g., 32 sectors of the compressed recording signal plus several linking sectors as a recording unit. The linking sectors are used to accommodate the additional signal generated by interleaving the compressed recording signal in the 32 sectors.
A recording and reproducing apparatus for a magneto-optical disc may employ one of several recording and reproduction modes for the compressed recording signal. In the CD-I and CD-XA formats, recording modes A, B, and C have been defined in which an uncompressed PCM audio signal, similar to that recorded on a normal Compact Disk (CD) but with a lower sampling frequency, is compressed to provide the compressed recording signal for recording on the magneto-optical disc. Recording mode A has a sampling frequency of 37.8 kHz, and the PCM audio signal is compressed by a compression ratio of two; recording mode B has the same sampling frequency as mode A and compression ratio of four; and recording mode C has a sampling frequency of 18.9 kHz, and a compression ratio of eight. In recording mode B, for example, the PCM audio input signal is compressed by a compression ratio of four, so that the playback time of a compact disc on which a mode B recording signal is recorded is four times theft of a disc recorded according to the standard CD format (CD-DA format). Using a recording mode in which the PCM audio signal is compressed enables the size of the recording and reproducing apparatus to be reduced, because a recording or playback time comparable with that of a standard 12 cm disc can be provided by a smaller-sized disc.
The velocity of the recording track relative to the pickup head (the "recording velocity") of the smaller-sized disc on which a recording mode B compressed signal is recorded is chosen to be the same as that of a standard CD. This means that the bit rate of the compressed recording signal reproduced from the disc is four times the bit rate required by the mode B decoder. This allows the same recording unit of the compressed recording signal to be read from the disc four times, but only one of the four readings of the recording unit of the compressed recording signal is fed into the decoder.
The compressed recording signal is recorded on the disc on a spiral track. When reproducing the track, the pickup head is caused to execute a radial track jump on each complete revolution of the disc. The track jump returns the head to its original position on the track. Causing the head to execute four track jumps causes the head to read the same part of the track four times. This method of reproducing the compressed recording signal recorded on the track is advantageous, especially when used in a small-sized portable apparatus, since it enables satisfactory reproduction to be obtained even if one of the four readings of the recording unit of the compressed recording signal is capable of being accurately decoded. This method of reproducing the compressed recording signal from the disc therefore provides a strong immunity against reproduction errors caused by physical disturbances and the like.
In future, semiconductor memories are expected to be used as a medium for recording digital audio signals. To enable semiconductor memories to provide a usable playing time, it is necessary to increase the compression ratio further by using variable bit rate compression encoding, such as entropy encoding. Specifically, it is anticipated that audio signals will be recorded and/or reproduced using IC cards employing semiconductor memories. A compressed recording signal that has been compressed using a variable bit rate compression technique will be recorded on and reproduced from the IC card.
Although it is expected that, in future, with progress in semiconductor technology, the playing time provided by an IC card will increase, and the cost of the IC card will decrease, compared with the playing time and cost of a present-day IC card, the IC card, which has barely started to be supplied to the market, is at present expensive and has a short playing time. Therefore, it is thought that an IC card might be used early on by transferring to it part of the contents of another, less expensive, larger capacity, recording medium, such as a magneto-optical disc. Signal exchange and re-recording operations would be conducted between the IC card and the magneto-optical disc. Specifically, a desired one or more selections recorded on the magneto-optical disc would be copied to the IC card. The copied selections would then be replaced by other selection(s) when desired. By repeatedly exchanging the selections recorded on the IC card, a variety of selections may be played on a portable IC card player using a small number of available IC cards.
Different applications require different bandwidths and signal-to-noise ratios for recording and reproducing audio signals. For example, when an audio signal is to be recorded and reproduced with high-fidelity quality, a bandwidth extending to 15 kHz or 20 kHz and a large signal-to-noise ratio are required. To provide these characteristics using a system in which a compressed digital recording signal is recorded on a recording medium and reproduced therefrom, the compressed recording signal must have a relatively high bit rate. For example, a bit rate in the range of 256 kbps to 64 kbps per audio channel is required. On the other hand, when a digital audio signal representing speech is to be recorded and reproduced, a bandwidth extending to 5 kHz or 7 kHz is more than adequate, and a lower signal-to-noise ratio may be acceptable. Such characteristics may be provided using a bit rate in the range of 64 kbps to several kbps. Lower bit rates increase the recording time of the recording medium. Thus, to record different types of audio signals while making optimum use of the recording capacity of the recording medium, the recording/reproducing apparatus should be capable of recording and reproducing at different bit rates as economically as possible.
Conventional recording and reproducing apparatus using, for example the above-mentioned recording modes A, B, and C operate at several different sampling frequencies to provide recording modes with different bandwidths and signal-to-noise ratios. To operate at different sampling frequencies requires a complex sampling frequency signal generating circuit, and increased complexity in the LSI signal processing circuits. Moreover, when the sampling frequencies of the compression modes are different, converting a compressed signal compressed in one compression mode to another compression mode is difficult.
When a compressed recording signal recorded on a high-capacity magneto-optical disc with a high bit rate is to be converted so that it can be recorded on a low-capacity IC card using a low bit rate recording mode, the compressed recording signal must be expanded back to an uncompressed PCM signal, which must then be re-compressed using a low bit rate compression mode to provide a low bite-rate recording signal. This requires a large amount of signal processing, which economically-viable signal processing LSIs may be unable to carry out in real time.
Additionally, in the low bit rate recording modes, the reduction in the number of bits available to represent the audio signal (the main information in the compressed recording signal) can lead to a deterioration of sound quality. If the bandwidth of bands into which the spectral coefficients are grouped is made the same at all frequencies, dividing the audio frequency range of 0 Hz to 22 kHz into 32 bands will make the bandwidth of each band approximately 700 Hz. This is many times the bandwidth of the low-frequency critical bands, which is typically about 100 Hz, and is larger than the critical bandwidth over most of the audio frequency range. This mismatch between the bandwidth of these equal bandwidth bands and the bandwidth of the critical bands at low and middle frequencies significantly reduces the efficiency of the compression process. Therefore, to reduce the bit rate of the compressed recording signal, it is necessary to reduce not only the amount of main information, but also the amount of subsidiary information.